Increased air quality regulations and strong market competition are forcing glass manufacturers to change the process of making glass. While post-combustion flue gas treatment techniques can solve the problem of pollution, they usually involve significant capital and operating costs making it more difficult for process improvements by the glass manufacturers to be economical.
One cost effective method for controlling emissions as well as reducing capital requirements is the implementation of oxy-fuel glass melting technology. Use of oxy-fuel in glass melting eliminates nitrogen in the melting process and reduces NO.sub.X and particulate emissions to below the levels set by the Environmental Protection Agency (EPA). In addition, oxy-fuel combustion reduces carbon dioxide emissions and brings numerous other benefits ranging from increased production capacity to savings in the amount of batch chemicals required.
Use of oxy-fuel burners in glass melting permits the burner designer to achieve varying flame momentum, glass melt coverage and flame radiation characteristics. Different burners produce different levels of NO.sub.X in furnaces where nitrogen is present from air leakage, low-purity oxygen supplied from a vacuum swing or pressure swing adsorption unit, nitrogen in the fuel, or nitrogen contained in the batch chemicals. Non-compliance with NO.sub.X emission standards, rules and regulations can lead to very large penalties and fines, substantial capital expenditure for clean-up technology, or require the purchase of NO.sub.X credits.
Conventional oxy-fuel burners used in glass melting have a significant problem in that the flame produced by the burner is relatively narrow and short providing very limited coverage of the molten glass in the furnace. Since such flames are at very high temperatures, areas immediately under those flames can easily overheat causing undesired side effects such as reboiling of the glass leading to the formation of scum on the melt surface. The scum on the melt surface is usually associated with poor heat transfer and inefficient melting operations. For some high quality glasses such as television panel and float glass, the glass quality can be significantly affected by the presence of scum in the furnace.
Another problem with conventional oxy fuel burners is related to the relatively low luminosity of an oxygen-natural gas flame. Radiation from such flames comes from the combustion products, water vapor and carbon dioxide, radiating predominantly wavelengths which are absorbed by the surface of the glass melt. This adversely affects the overall heat transfer as this surface absorbed heat is re-radiated not only where it needs to go, i.e. down into the lower layers of the glass melt, but also back up towards the furnace crown. In contrast, luminous flames radiate a significant portion of radiation in the wavelengths that penetrate glass, thus making it easier to deliver heat to the lower layers of the melt.
Another problem associated with the use of oxy-fuel burners is that they operate at relatively high momentum, i.e. flame velocity, which can increase volatilization of volatile batch components and increase particulate emissions. Such burners can also increase refractory corrosion due to higher refractory temperatures and higher volatile concentrations in the gas phase. U.S. Pat. Nos. 5,199,866; 5,256,058; and 5,346,390 disclose methods and devices for producing luminous flames at lowered flamed momentums. However, even with the advent of the patented burners and processes, flame radiation, flame coverage and NO.sub.X caused by leaky furnaces have not been fully addressed.
U.S. Pat. No. 4,927,357 discloses a gas-injection lance, burner which produces a flame by having an elongated fuel jet which entrains air from a port above the fuel jet intersecting an elongated gas (oxygen) jet inside a furnace to produce a flame flattening effect.